Roller cone drill bits create an uncut region on the bore hole bottom known in the art as "uncut bottom." This is the region on the bore hole bottom that is not contacted by the primary row cutter elements. Primary row cutter elements are the cutting elements that project the furthest from the cone body for cutting the bore hole bottom. If this uncut area is allowed to build up, it forms ridges. As used herein, the term "ridge" means the uncut formation material that remains between the kerfs cut by adjacent rows of cutter elements as the bit is rotated in the borehole. In some drilling applications, ridges are not significant, because the formation that would form the ridges is easily fractured and ridges do not tend to build up. By contrast, in rock formations that are not easily fractured, or when the formation becomes plastic under the high down hole pressure, ridges tend to build up. The formation of ridges is detrimental to the drill bit, as it causes wear on the cone body and cutter elements, and slows the drill bit rate of penetration.
The increasing use of down hole motors with bent housings and/or bent subs in the drill string assembly for directional drilling introduces a wear characteristic where the outer surface of individual cutter elements becomes heavily worn, while the inner surface reflects relatively little wear. As used herein, "outer surface" refers to the side or edge of the cutter element that is closest to gage when the cutter element is at its closest approach to the side wall. Correspondingly, as used herein "inner surface" refers to the side of the cutter element that is closest to the bit centerline when the cutter element is at its closest approach to the side wall. This wear characteristic is particularly caused by the drilling application wherein the drill string is rotated and a bend is employed in the motor housing, which typically can have an angle from 1 to 3 degrees. This causes the circumference of the borehole to increase and causes the ridges that are formed on the borehole bottom to be circumferentially longer than those formed by a bit used without a bent motor housing attached to the drill string assembly. If the ridges are not fractured, the outer surface of the cutter elements encounters increased lateral loads. This leads to excessive wear on both the cutter elements and the cone body. This excessive wear will ultimately lead to breakage or loss of the cutter elements.
Furthermore, the flow of high pressure, abrasive fluid (drilling mud) out of and across the face of the bit causes high rates of bit erosion, particularly in areas where fluid flow is relatively rapid. Channeling of the fluid between cutter elements and recirculation of the fluid around the cutter elements can result in localized rapid fluid flow and undesirable localized erosion.
Hence, it is desired to provide a drill bit that ensures the fracture of the ridges and thereby decreases the wear on the outer surfaces of the cutter elements and on the cone body. It is further desired to provide a bit that mitigates the erosive effect of channelized fluid flow on the bit.